The use of some form of anesthesia, or “reversible lack of awareness”, can be dated as far back as the Greek and the Persian empires. However, it was not until the 19th century that modern narcotic anesthesia agents would be discovered. The introduction and the development of effective anesthetics in the 19th century was, with Listerian techniques, one of the keys to the development of successful surgical protocols.
Although anesthesia has made modern surgical procedures possible the process has, from the beginning, been fraught with danger. For example, the first chloroform surgeries were performed in late 1847, and the first fatality directly attributed to chloroform anesthesia was recorded mere months later in January 1848. The framed physician John Snow published a classical study of chloroform deaths as early as 1858. Indeed, medical literature is replete with the names of prominent figures in medicine that wrote about the dangers of anesthesia. Harvey Cushing, generally considered the father of modern neurosurgery, lost a patient who aspirated gastric contents during ether anesthetic. He later wrote that this event almost caused him to leave medical school. As a result, anesthesiologists the world over have long been concerned with and involved in studies related to anesthesia safety. Many prominent anesthesiologists in the United States began to collect statistics of anesthetic morbidity and mortality as early as the 1930's. However, the dangers of anesthesia are not purely historical. Indeed, it was not until 1983 that a modern system of reporting was implemented so that anesthesiologists could develop statistics and define the parameters of future studies.
The results of these studies have been to create standards and protocols for how anesthesia is administered and how a patient is monitored while under anesthesia. For example, under current protocols patients being treated under general anesthetics must be monitored continuously to ensure the patient's safety. For minor surgery, this generally includes monitoring of heart rate (via ECG or pulse oximetry), oxygen saturation (via pulse oximetry), non-invasive blood pressure, inspired and expired gases (for oxygen, carbon dioxide, nitrous oxide, and volatile agents). For moderate to major surgery, monitoring may also include temperature, urine output, invasive blood measurements (arterial blood pressure, central venous pressure), pulmonary artery pressure and pulmonary artery occlusion pressure, cerebral activity (via EEG analysis), neuromuscular function (via peripheral nerve stimulation monitoring), and cardiac output. In addition, the operating room's environment must be monitored for temperature and humidity and for buildup of exhaled inhalational anesthetics, which might impair the health of operating room personnel. While these protocols have resulted in significant improvements in anesthesia related mortality, they have not eliminated all of the risks associated with anesthetized and otherwise consciousness altered individuals.
For example, one risk that has become more prevalent over time is perioperative visual loss. Perioperative visual loss (POVL) broadly refers to permanent impairment or total loss of sight associated with general anesthesia. The relevant perioperative period generally includes a time from the immediate preoperative assessment through discharge from the acute healthcare facility, and is indicated for patients who, within seven days following non-opthalmological surgery, began to develop visual impairment and/or blindness. Despite the data and the in-depth demographic studies available, the mechanism of perioperative ischemic optic neuropathy is still theoretical and, thus, up until this time, monitoring and prevention strategies could not be effectively defined.
Another potentially disturbing complication can be ‘anesthesia awareness’. In this situation, patients paralyzed with muscle relaxants may awaken during their anesthesia, due to decrease in the levels of drugs providing sedation, lack of awareness and/or pain relief. If the anesthesia provider misses this fact, the patient may be aware of his surroundings, but be incapable of moving or communicating that fact.
Neurological monitors are becoming increasingly available which may help decrease the incidence of POVL and awareness. One exemplary monitor that is currently available is the BIS monitor, manufactured by Aspect Medical Systems of Natick, Mass. The BIS device monitors EEG-based brain function to reduce the incidence of recall or awareness of a patient while under anesthesia. During function the BIS monitor uses proprietary algorithms to monitor brain activity and give the anesthesiologist a series of empiric numbers upon which to assess the patient's level of consciousness. While monitoring EEG-based brain activity has been shown to be of some benefit in allowing for quicker recovery from anesthesia, studies have indicated that the EEG measurements can be dramatically impacted even when the patient is not under anesthesia. For example, in a study published in 2008 it was shown that BIS scores could be changed by as much as 20 basis points simply by the administration of muscle relaxants to non-anesthetized patients. (Lu, et al., Int. Anesthesia Res. Soc., 107:4, 2008, the disclosure of which is incorporated herein by reference.) Likewise, a study published in the New England Journal of Medicine showed that awareness under anesthesia occurred in patients even when the BIS values were within target ranges. (Avidan, M. S., et al., New England Journal of Medicine, 358, 1097, 2008, the disclosure of which is incorporated herein by reference.)
A second device being marketed for use as an anesthesia monitor is the BAER system, which stands for Brain Auditory Evoked Response. These monitors have been used successfully in the past to evaluate brain injury; however, recently it has been suggested that such a technique could be used to monitor the depth of anesthesia. Recent studies have openly questioned this assumption. However, some studies on the efficacy of the system have shown that there is no correlation between the potentials measured by the BAER system and level of anesthesia.
In summary, despite the widespread marketing of these new monitoring devices many case reports exist in which awareness under anesthesia has occurred despite apparently adequate anesthesia as measured by such neurologic monitors. Accordingly, a method and apparatus for monitoring a patient under anesthesia that could provide better data concerning both POVL and Anesthesia Awareness is needed to provide surgeons with adequate information to enable real-time prevention of these serious conditions.